The present invention relates to a catalytic system based on molybdenum, phosphorus, vanadium and oxygen; it has more particularly for its subject a catalytic system consisting of a heteropolyanion to which are connected, in variable proportions, cations such as protons H.sup.+, the vanadyl cation VO.sup.2+ or metal cations such as copper: Cu.sup.2+, iron: Fe.sup.3+ or cobalt Co.sup.2+. The catalytic system which is the subject of the invention is particularly suitable for the oxydehydrogenation reaction of saturated carboxylic acids or esters to the corresponding unsaturated acids or esters, in particular for the oxydehydrogenation reaction of isobutyric acid to methacrylic acid, as well as for oxidation reactions such as the oxidation of methacrolein to methacrylic acid.
Catalytic systems have been known for a long time which make it possible to carry out the oxydehydrogenaion of saturated carboxylic acids or esters to the corresponding unsaturated acids or esters. Thus, there is described in French Pat. No. 2,497,795 a catalyst of general formula: FeP.sub.x Me.sub.y O.sub.z, in which Me is Li, Na, K, Rb, Cs, Mg, Sr or Ba: such a catalyst is suitable for the manufacture of methacrylic acid from isobutyric acid. The synthesis of methacrylic acid from isobutyric acid in the presence of a binary mixture of oxides consisting of vanadium oxide (V.sub.2 O.sub.5) and phosphorus pentoxide (P.sub.2 O.sub.5) has also been described (J. Catal. 98, 401, 1986). In this same publication, comparative tests were also carried out starting from catalysts of formula H.sub.5 PMo.sub.10 V.sub.2 O.sub.40. The results indicated show that the selectivity towards methacrylic acid levels out at around 60 % whatever the catalyst used.
Catalytic systems based on heteropolyanions have also been described in other publications as catalysts for the oxidising dehydrogenation reaction of isobutyric acid, or of its esters, to methacrylic acid or to its esters.
Thus, German Application (DOS) 2,722,375 describes catalysts obtained by concentration of an aqueous solution of a heteropolyacid of composition H.sub.5 PMo.sub.10 V.sub.2 O.sub.40 and of a copper salt, optionally in the presence of an alkali metal salt, the dry residue then being calcined. Such catalysts make it possible to achieve selectivities towards methacrylic acid of 71-72 % and, by incorporation of lithium, it is possible to achieve 74.8 %.
U.S. Pat. No. 4,307,247 proposes, for the same oxidising dehydrogenation reaction, catalysts based on molybdenum, vanadium, copper, phosphorus and bismuth, to which an alkali metal of the potassium, rubidium or caesium group, and optionally other metals, is added as well as oxygen in determined quantitative ratios.
U.S. Pat. No. 4,314,075 proposes, for the same reaction, similar catalysts based on the following elements: molybdenum, vanadium, copper and phosphorus and an alkali metal chosen from potassium, rubidium or caesium and one or more other metals as well as oxygen in determined quantitative ratios. All metals are present during the hydrothermal reaction which leads to the aqueous solution of a heteropolyanion. The selectivities for methacrylic acid achieved in the presence of these catalysts are between 60 and 70 %.
U.S. Pat. No. 4,522,9 34 also describes catalysts for this oxydehydrogenation reaction, of general formula H.sub.3+x PV.sub.x W.sub.3-x Mo.sub.9 O.sub.40 with 0&lt;x&lt;3. The system H.sub.3.5 PMo.sub.9 V.sub.0.5 W.sub.2.5 O.sub.40, supported on silica, makes it possible to achieve a conversion of isobutyric acid of 93 % and a selectivity for methacrylic acid of 79 %.
All these catalytic systems derived from heteropolyanions, however, exhibit the general drawback of having a limited stability over time under the oxydehydrogenation reaction conditions.
It is also recorded in the prior art that this type of catalyst can be used to carry out the oxygenation of methacrolein to methacrylic acid with good yields. Proc. 7th Intern. Congress Catalysis, Tokyo (1980), p. 755.
It is thus felt necessary to have available catalysts of the heteropolyanion type which make it possible to obtain selectivities towards unsaturated carboxylic acids which are industrially acceptable in combination with high conversions of the starting saturated carboxylic acids which are subjected to the catalytic oxydehydrogenation reaction; these catalysts must also exhibit good degrees of conversion of aldehydes in the oxidation reactions of aldehydes to acids, in particular during the oxidation of methacrolein to methacrylic acid. Moreover, it is also necessary that these catalysts remain stable over time under the reaction conditions.